1. Field of the Invention
The present invention relates to a method and a laser oscillator for the generation of a laser beam.
Although not exclusively so, this method is particularly suitable for autonomous generation of a laser beam (that is to say without an initiation beam being injected in advance).
2. Description of the Related Art
As is known, for generating a high-power laser beam it is known initially to generate a plurality of elementary laser beams, and subsequently to aggregate the beams thus generated so hat they combine coherently at a single focus point and thus form a single laser beam, which has a higher power the higher the number of elementary beams.
However, for the power of the resulting laser beam to be optimal, it is important for the elementary laser beams all to be of the same emission frequency and the same phase.
A known solution for achieving this is disclosed for example in document U.S. Pat. No. 6,366,356. It involves initially generating a “master” laser beam from a laser oscillator, and subsequently space-division multiplexing the master beam into a plurality of elementary laser beams at the output of this oscillator. These elementary beams are amplified separately, in parallel, by the same number of optical amplifiers, and subsequently the phases thereof are adjusted. These elementary beams which are thus placed in phase are finally combined to form a single laser beam having a higher power than the master laser beam. During this combining, since all of the elementary laser beams come from the same master beam, they actually have the same emission frequency and the same power. Further, as a result of adjusting the phase of each elementary laser beam, they are also in phase.
However, this solution has the drawback of requiring two optical modules in succession: first a “master” laser oscillator and then a set of amplifiers in paralleI. Finally, the device is completed by a phase analysis system for the elementary beams, a feedback loop, and a set of phase modulators located between the master oscillator and the amplifiers. It goes without saying that a device of this type proves to be particularly complex and expensive to implement, while taking up a significant amount of space.
For all of these reasons, it is desirable only to have a laser oscillator, which can itself generate the elementary laser beams autonomously for the subsequent aggregation thereof.
A known solution for achieving this is disclosed in document U.S. Pat. No. 7,130,113. Specifically, it involves arranging, within a single laser oscillator, a plurality of elementary laser cavities which can respectively generate a plurality of elementary laser beams having identical emission frequencies. For this purpose, each elementary laser cavity comprises an amplification medium which, when it is pumped or excited by introducing energy, generates an intrinsic noise in all spatial directions. The light which propagates the direction corresponding to the axis of the elementary laser cavity is amplified every successive time it passes through the amplification medium of each laser cavity, so that each elementary laser beam converges to a stationary state in which it reaches the nominal power thereof at the output of the laser oscillator. Subsequently, the elementary laser beams thus generated in the different elementary laser cavities are placed in phase in an all-optical manner, using a diaphragm which limits the emitted elementary laser beams to portions dose to the optical axis. Finally, the elementary laser beams thus placed in phase are combined so as to form a single high-power laser beam. Placing the elementary laser beams in phase is thus made simpler.
However, although this known solution is effective for combining a limited number of elementary beams (typically fewer than ten), it is not satisfactory when the number of elementary beams is larger. Specifically, as is known, the resonant frequencies (or modal coincidences) of the set of elementary laser cavities are the degrees of freedom of the laser oscillator, and make it possible to adjust the laser emission spectrum so as to maximise the effectiveness of combination. It is also important to have the largest possible number thereof, so as to combine the elementary laser beams effectively. However, when the number of elementary beams increases, these resonant frequencies are found to become scarcer, and this actually reduces the effectiveness of combination. Therefore, when it is attempted to place elementary laser beams in phase passively, and therefore by self-adjusting the emission spectrum, as is the case in document U.S. Pat. No. 7,130,113, the modal coincidences become scarcer and the effectiveness of combination is reduced.